"Redox" batteries function as a bulk energy storage system of electric energy and have a very high overall energy efficiency as compared to many other systems. The storage of energy takes place through the solutions of metal ion pairs at differing states of oxidation. If two such ion pairs, whose "redox" potentials deviate sufficiently far from each other, are allowed to react on two different electrodes which are separated from each other by a membrane, then a potential difference is obtained, i.e. electric energy (See, for example, EP-P 0 143 300).
"Redox" pairs of the above mentioned kind are, for example Fe.sup.3+ /Fe.sup.2+ and Cr.sup.3+ /Cr.sup.2+. In cells containing such "redox" pairs, for example Fe/Cr "redox" cells, the storage of power-upon applying the charging voltage-takes place through the following processes: ##STR1## The values of the potentials refer to the standard hydrogen electrode. At the positive electrode bivalent iron ions are oxidized while at the negative electrode trivalent chromium ions are reduced. On reversing these processes-reduction of (*) oxidation of bivalent chromium ions (Cr.sup.2+) at the negative electrode-the stored electric energy is again freed and the discharge voltage can be tapped at the electrodes. Since the (*) trivalent iron ions (Fe.sup.3+) at the positive electrode and discharge voltage of a single cell is generally too low for technical applications, several "redox" cells are always electrically connected in series by which "redox" batteries result.
"Redox" cells generally are operated with acidic electrolyte solutions, for example with solutions of iron chloride or chromium chloride in hydrochloric acid. The electrochemical reactions take place at electrodes which are inert to the particular electrolyte solutions, i.e. cathode fluid (catolyte) or anode fluid (anolyte). Coal or graphite, which are inert to the anode and cathode fluids, generally serve as electrode material. (See, for example U.S. Pat. No. 3,996,064 and U.S. Pat. No. 4,382,116).
The "redox" reaction of the iron in "redox" cells normally does not present any problems. It takes place particularly when graphite materials are used, at a sufficient rate and with a current yield of nearly 100%. In contrast, the "redox" reaction of the chromium presents problems since it is not catalyzed to a sufficiently great extent by graphite. For that reason, previous attempts have been made to activate the graphite electrodes for the chromium reaction.
A suitable measure for activating the chromium electrode is by coating with metals. For example, gold catalyzes the oxidation of bivalent chromium and the reduction of trivalent chromium so that a sufficient reaction speed can be achieved for the discharge reaction as well as for the charge reaction. However, the hydrogen overvoltage on gold is less than on graphite. Therefore, during the charging process in this case, as a side reaction of the reduction of the trivalent chromium, an undesirable hydrogen formation, occurs. Thus, the current yield of the charging process for the chromium electrolyte is markedly below 100%. As a consequence, the charging state of the chromium solution with each cycle remains further behind the charging state of the iron solution.
The difference between the charging state of the chromium solution and that of the iron solution must be balanced externally with a special cell. (See, for example: M. A. Reid and L. H. Thaller "Improvement and Scale-up of the NASA Redox Storage System", DOE/NASA/12726-6, NASA TM-81632, 1981). The concepts developed until now for such a "rebalance cell," however, use up energy so that the energy efficiency of the "redox" system is diminished. The higher the rate of hydrogen development at the chromium electrode and, hence, the required power of the rebalance cell, the lower the energy efficiency of the energy storage system.
A further problem arises in this connection with respect to the material balance. The hydrogen formed as a byproduct in the chromium reduction represents a substance loss which, in the interest of a long operating life of the total system, must be compensated for in order to permit a closed cycle.
In order to decrease the hydrogen formation, attempts have been made to use a second catalyst besides gold. Known for this purpose are lead and cadmium (See, for example U.S. Pat. No. 4,192,910 and U.S. Pat. No. 4,270,984) as well as thallium (See, EP-P 0 137 990). However, when applied in practice, the hydrogen overvoltage is also insufficient to suppress the hydrogen development to a sufficient extent.
It is the object of the invention to implement a battery of "redox" cells of the aforementioned type with a catalyst for accelerating Cr.sup.3+ /Cr.sup.2+ "redox" processes in such a way that the current yield of the "redox" reaction of the chromium is optimized and, hence, maximum possible energy efficiency is achieved for the overall system.